Even swans, who are known to mate for life, are unfaithful partners, according to researchers.

Social scientists claim that there is not one culture in which adultery doesn't happen. In the United States, some reports say that one in three married men will cheat at some point; for women, the rate is one in four.

It is a thorny, contentious issue that brings along with it a multitude of questions. Are people born to cheat? Is the seventh commandment still relevant in a country where more than 40 percent of the marriages end in divorce? What constitutes adultery? Is lying worse than cheating?

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To explore all of these questions, "Nightline" went out to the heart of the Bible belt for the fourth installment of the "Nightline Face-Off" series, moderated by co-anchor Cynthia McFadden, asking simply: Are we born to cheat?

It was a packed house. More than 3,000 people -- mostly church members -- showed up to hear the different ideas from the four panelists, all of whom said they are happily married, but who had very different ideas about what constitutes cheating.

God Wants Us to Have the Best Sex

The conversation was a powerful, candid and, at times, painful look at adultery and marriage, and the panel was as complex as the topic.

Pastor Ed Young is a married father of four and senior pastor of the Grapevine, Texas-based Fellowship Church, where the debate took place. Young has written and preached extensively on the topic of marriage and sex, and even has been known to challenge his married congregants to engage in seven days of sex.

"God is pro-sex," Young told "Nightline" prior to the debate. "He thought that up, and He wants us to have the best sex possible. And that is in a marriage -- one man, one woman together. And that's the deal. And I know that a lot of people have slept in the wrong bed. God wants to forgive, but also His ideal is that marriage bed."

Joining Young was Jonathan Daugherty, who identified himself as a recovering sex and porn addict and admitted committing adultery. Daugherty is now back with his wife, with whom he has created the San Antonio-based Be Broken Ministries, where they counsel others suffering from sexual addictions.

"[I] have lived on both sides," he said. "So I hope to be able to put my life on the scale and say this is my experience and this is why I have come to the conclusion I have come to regarding marriage and it's importance."

Daugherty admitted that discussing his personal life in such a public forum is tough.

"You have to bring up some things in the past that were clearly mistakes that I made, and it's not fun to bring those things up," he said. "But I look at it in the sense that if I can help someone who is struggling with those things to come to a conclusion that might improve their life or get the help that they need, then I think it's worth going ahead and putting myself out there and telling people about it."

'Life is Short; Have an Affair'

On the other side of the issue was Noel Biderman, from Toronto, Canada, who said he's the happily married father of two. Biderman is also the president and CEO of Ashley Madison, a Web site for people who want to have an affair that claims 4.5 million subscribers. The group's slogan sums up its mantra: "Life is short. Have an affair."

"You know, people ask me all the time why and how Ashley Madison is so successful, and the answer is simple," he said. "Infidelity is common, it's rampant, it is in 60 percent of American homes. But truthfully, it can be a positive. It can save the institution of marriage, and that's what I intend to argue tonight."

Debating alongside Biderman was author Jenny Block, who admitted to cheating on her husband of 12 years, calling that period of time the "ugliest, most awful six months of my life."

Block now lives with her husband and daughter in Texas, but also has a female lover. She has an open marriage and described her relationships as polyamorous. She has written candidly about her life and her loves in her book, "Open: Love, Sex, and Life."

"I dont believe that what I'm doing is committing adultery, because everyone knows what's going on," she said. "We consider our relationship an open marriage. I dont think there's anything wrong with outside sexual relationships in marriage, [but] I do think there is a lot wrong with lying inside of a marriage."

Pastor Ed: Have an Affair With ... Your ... Wife

From the start, the debate became contentious as each side revealed its personal and controversial positions.

Young kicked things off by encouraging Christians not only to engage in sex, but to enjoy it.

"Primarily, [sex is] for pleasure. Secondarily, it's for procreation," he said. "And when you have a man and a woman making love, then they're going to hit on all cylinders. And when you don't, when you step out of that relationship, the result is going to be chaos. I like Noel's line: 'Life is short.' OK, 'Have an affair,' but I would add three words: with ... your ... spouse."

Block followed by discussing her affair.

"[My husband] wasn't so upset with my having sex with someone else, but was very upset with my lying to him about it," she said. "And so for me, I feel like relationships, marriage, the whole shebang is, number one, about honesty. Sex, yeah, it's a great part of a relationship. But I'm not convinced that it's the cornerstone of a marriage or of any relationship."

Daugherty, who suffered from a secret sex addiction that spanned two years, also detailed his personal struggle with cheating.

"Before AshleyMadison.com was around, I was using the Internet to try to find [and] set up anonymous sexual encounters," he said. "And that just started to crumble not only my own integrity, [but] it crumbled any sort of communication and trust that was in our marriage. Obviously, I'm going to come at it from the standpoint that God is the inventor of marriage. He's the inventor of sex and so he gets the prerogative to define what the terms are. And his definition was one man with one woman for life."

Biderman Defends Web Site: What Kind of Pastor Are You?

When questioned about the perceived innocence of lust, Young shot back that lust is a form of adultery.

"Lust is a God-given desire that's gone haywire," he said. "And I would say the sexual desire in marriage can light that fire that keeps the marriage together."

But in the case of lust, Young said, this desire is wrong.

"You look at someone -- you think and you picture having sexual relations with that person," he said. "Too many people unknowingly, ignorantly, say, 'You know what, we're just dogs in heat. We can't resist it. We're going to do what we do. Boys will be boys, girls will be girls.'"

Young also took aim at Biderman's Web site, saying, "What if I supplied all the bomb-making materials to terrorists worldwide, but I said, 'Hey, I'm not a terrorist.'"

Young continued: "I deal with the broken homes. I deal with the children whose lives are up for grabs because someone is selfish enough to step outside the marriage and do what they want to do."

Biderman shot back in defense of his users.

"What I hear all the time is, 'Why cheat if it's not working for you, just leave.' That's the selfish act. Walking away from your family to pursue your own sexual needs, that's a sexual act," he said. "And so, what I hear from my members all the time is they're in sexless relationships, they've tried talking about it. And so rather than leave, they would rather do this. If you can't understand people in those positions, I don't really know what kind of pastor you are because they are suffering."

Former Sex Addict: 'We Dropped the Ball on This'

As the debate continued, Daugherty spoke up regarding the role God plays in marriage, adultery and divorce.

"I'll say, too, that we have to remember that God didn't mess up the institution of marriage. God created it perfectly. So whenever we're talking about kind of revisiting marriage and these sorts of things, it's because, like Ed said, we dropped the ball on this," he said.

Block, who said she thinks adultery is a terrible thing, also isn't sold on the ideal of marriage.

"I don't think it's a bad idea," she said. "But I think the way we've designed it, this Cinderella fairy tale -- happily ever after -- that 40 percent of people fail at, and one out of three men cheat on, it doesn't work for everyone. It works for some people fabulously! Some of my very best friends are monogamists. But it doesn't work for everyone."

After almost two hours, and after the audience asked the panelists questions and discussed their own battles with adultery, the debate came to a close.

"We need to not be afraid to have these sorts of discussions," said Daugherty. "You know, sometimes I have to admit, in some of our Christian circles, we have a tendency to just kind of get in our little bubbles, our cocoons. And were unwilling to engage culture. Were unwilling to even entertain other ideas and notions, because were so afraid that if everybody doesnt agree with us, then everything is lost. Well folks, everybody is not going to agree with us. And that's OK. But lets be open enough to be able to have conversations and dialogue so that we are not intolerant of other people."

"Really thoughtful questions and I couldnt have asked for more, so thank you for hosting us," said Biderman. "Again, I think I agree with my friends over here, that its our choice [to be monogomous]. I've chosen for myself to be in a monogamous relationship. But I know that it's about my effort and nothing else. And if it went wrong it would be my fault."

Young then closed the debate with a prayer, praising his fellow panelists, and asking God to bless the lives of both Block and Biderman.